ethtool: assert that drivers with sym hash are consistent for RSS contexts

ethtool: assert that drivers with sym hash are consistent for RSS contexts

Supporting per-RSS context configuration of hashing fields but
not the hashing algorithm would complicate the code a lot.
We'd need to cross check the config against all RSS contexts.
None of the drivers need this today, so explicitly prevent
new drivers with such skewed capabilities from registering.
If such driver appears it will need to first adjust the checks
in the core.

Link: https://patch.msgid.link/20250717234343.2328602-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>